The present invention is directed to breathable thermoplastic films. Such materials have a wide variety of uses, especially in the areas of limited use and disposable items.
Films have been traditionally used to provide barrier properties in limited use or disposable items. By limited use or disposable, it is meant that the product and/or component is used only a small number of times or possibly only once before being discarded. Examples of such products include, but are not limited to, surgical and health care related products such as surgical drapes and gowns, disposable work wear such as coveralls and lab coats and personal care absorbent products such as diapers, training pants, incontinence garments, sanitary napkins, bandages, wipes and the like. In personal care absorbent products such as infant diapers and adult incontinence products, films are used as the outer covers with the purpose of preventing body wastes from contaminating the clothing, bedding and other aspects of the surrounding environment of use. In the area of protective apparel including hospital gowns, films are used to prevent cross exchange of microorganisms between the wearer and the patient.
While these films can be effective barriers, they are not aesthetically pleasing because their surfaces are smooth and either feel slick or tacky. They are also visually flat and "plasticy" thereby making them less desirable in apparel applications and other uses where they are in contact with human skin. It would be more preferable if these items were more cloth-like from both a tactile and visual standpoint. For example, infant diapers that have the feel and appearance of traditional cloth undergarments are perceived as premium products and may, in some cases, overcome the tendency to believe that they need to be covered by outer garments for aesthetic reasons. Garment-like adult incontinence products could improve the self-image of the incontinent individual. In addition, more garment-like isolation gowns would help the hospital environment feel less foreign and threatening to the patient and increase the comfort of the wearer. It is also preferable to have films that can make an outercover material with more elastic give and recovery to provide better fit and comfort.
Lamination of films has been used to create materials which are both liquid-impervious and somewhat cloth-like in appearance and texture. The outer covers on disposable diapers are but one example as can be seen in coassigned U.S. Pat. No. 4,818,600 dated Apr. 4, 1989 and U.S. Pat. No. 4,725,473 dated Feb. 16, 1988. Surgical gowns and drapes are other examples. See, in this regard, coassigned U.S. Pat. No. 4,379,102 dated Apr. 5, 1983.
A primary purpose of the film in such laminations is to provide barrier properties. There is also a need for such laminates to be breathable so that they have the ability to transmit moisture vapor. Apparel made from laminates of these breathable or microporous films are more comfortable to wear by reducing the moisture vapor concentration and the consequent skin hydration underneath the apparel item. However, the pore size in breathable films cannot be too large, especially in protective apparel applications where chemical vapor penetration presents a contamination risk to the wearer.
The conventional process for obtaining a breathable microporous film has been to stretch a thermoplastic film containing filler. Microvoids are created by the filler particles during the stretching process. The film is usually heated prior to these drawing processes to make the film more plastic or malleable. This drawing or stretching also orients the molecular structure within the film which increases its strength and durability. The molecular orientation caused by stretching is desired to improve durability.
A film can be stretched in the machine-direction or the cross-machine direction. Stretching the film in the cross direction is particularly challenging because forces must be applied to the edges of the film to cause it to elongate width-wise. Tenter frames are commonly used. In contrast, stretching the film in the machine direction is relatively easy. It is only necessary to increase the draw, or speed ratio, between two rollers while the film is in the heated or plastic state. There is a durability problem, however, with uni-directionally-stretched films, be it machine direction or cross-direction. Uni-directional stretching causes molecular orientation only in the stretched direction. This results in films that are easily torn or split along that dimension. For example, a machine-directionally oriented film has a propensity to split or tear along the machine direction. Also, the tensile characteristics of the (machine-directionally stretched) film are dramatically increased in the machine direction, but the tensile strength in the cross-direction is significantly inferior to that of the machine direction. Thus, for example, if at the same time that the CD strength of the film decreases, the CD break elongation is also reduced, the film can split very easily in use, and an article made with it, such as a diaper, may leak, obviously an undesirable effect.
These durability problems with uni-directionally stretched or oriented films are well known. Two approaches are commonly used to obviate the product durability problems resulting from these highly isotropic strength characteristics. The first is to stretch-orient the film in both the machine and cross direction. Films that have been biaxially stretched have more balanced strength properties. The second approach is to combine into a laminate one layer of machine directionally oriented film with one layer of cross-directionally oriented film.
One other manufacturing issue is the strength of "aged" films. In commercial manufacturing operations, "fresh" films such as newly extruded films are generally not available for orientation. Extruded films are often set aside or stored for later orientation, usually at room temperature. During this storage period, a change in morphology of the polymer may occur, which change could be the cause of film property changes. Orientation of such aged film often results in products with lower durability characteristics such as lower CD Peak Strain (or cross directional break elongation), a critical property, for example, for the durability of a diaper outer cover made from this film.
There is therefore a need for an elastic breathable film and process that provides a film with both the cloth-like aesthetics and the durability and comfort that are desired.